1. Field of the Invention
The present invention relates to an electric power steering apparatus. More particularly, it proposes an improved electric power steering apparatus.
2. Description of Related Art
The electric power steering apparatus serves to detect the steering torque and to assist the steering force by the rotation of an electric motor which is driven according to the detected torque.
FIG.1 is a block diagram showing an outline construction of a conventional electric power steering apparatus. The output of a torque sensor 2 provided at the steering shaft (not shown) is inputted in a CPU 1 through an AMP (amplifier) 3, wherein first the A/D conversion is carried out. The CPU 1 stores in its memory a table of the relation between the detected torque values and the target values of the drive current for an electric motor 5, and reads out the target current corresponding to the input torque. In the CPU 1, the deviation is calculated between the read-out target current and the feedback drive current of the motor 5 which is detected by a drive current detecting resistor and a drive current detector circuit 9 to be inputted in the CPU 1. The deviation is dealt in PI calculation, thereby the control variable is calculated, so that the PWM wave signal corresponding to the variable and the drive control signal indicating the rotary direction of the motor 5 are made out to be given to an FET driving circuit 4.
The motor 5 is connected to four switch transistors 8a, 8b, 8c, 8d in a H-bridge and the FET driving circuit 4 outputs a gate signal respectively to make the transistors 8a, 8d "on" for the leftward rotation and to make the transistors 8b, 8c "on" for the rightward rotation. Incidentally, the transistors 8c, 8d are always "on" for its rotation and the transistors 8a, 8b are driven by the PWM wave signal.
A drive inhibition circuit 7 is provided for a countermeasure against the runaway in the CPU 1, and the outputs from AMP 3 are respectively given to the minus input of a comparator 71 and the plus input of a comparator 72. The plus input of the comparator 71 and the minus input of the comparator 72 are respectively given threshold voltages VR, VL corresponding to R, L shown in FIG.2 by a voltage divider 73.
FIG.2 shows a relation between the detected torque (axis of abscissas) and the drive current (axis of ordinates) outputted from the CPU 1 for the motor, and represents that the rightward drive of the motor 5 is inhibited in case of the detected torque not greater than R including the dead zone and that the leftward drive is also inhibited in case of the torque not smaller than L. The outputs from the comparators 71, 72 are respectively given to the bases of switch transistors 74, 75. The emitters of the transistors 74, 75 are at earth potential and the collectors are respectively connected to the gates of the transistors 8c, 8d.
Consequently, when the output of the detected torque from AMP 3 becomes lower than VR, the output from the comparator 71 becomes "H" level, and the transistor 74 is turned "on", thereby the gate of the transistor 8c is forced to the "L" level, so that the rightward rotation of the motor 5 is inhibited despite of the output of the FET driving circuit 4. When the output from AMP 3 becomes higher than VL, similarly the gate of the transistor 8d is forcibly made to the "L" level, thereby the leftward rotation of the motor 5 is inhibited. Thus, in the zone where the direction of the detected torque and the direction of the motor drive are different, the operation of the motor is inhibited so that no conflict arises.
Next, FIG.3 is a block diagram showing the outline construction of the conventional electric power steering apparatus wherein the operation within the CPU 1 is clarified. In FIG.3, the same parts as those in FIG.1 are designated with the same numbers. The output of the torque sensor 2 provided at the steering shaft (not shown) is inputted through an interface 43 into the CPU 1, wherein first the A/D conversion (51) is carried out, and next the phase compensation (52) is performed. The CPU 1, stores in its memory a target current table (53) showing the relation between the detected torque values and the target values of the drive current for an electric motor 5, and reads out the target current corresponding to the input torque. The output of the A/D conversion is also differentiated (58), and the result is added to the target current (54). This differential value is added for the inertial compensation of the motor 5.
The deviation is calculated (55) between the result in the above-described addition and the feedback drive current of the motor 5 which is detected by the drive current detector 9 to be subjected to A/D conversion (59) in CPU 1. The deviation is dealt in PI operation (56), thereby the control variable is calculated, so that the PWM wave signal corresponding to the variable and the drive control signal indicating the rotary direction of the motor 5 are made out (57). Those signals are given to a driving circuit 40 constructed as an H-type bridge composed of the motor 5 and the four switch transistors. Although not illustrated here, the vehicle speed information detected by the vehicle speed sensor is also inputted into the CPU 1, and the table of the target currents corresponding to the information is read out.
The operation of the drive inhibition circuit 7 provided as the countermeasure against a runaway in the CPU 1 is the same as the above-described conventional case, and its operational characteristic is shown in FIG.2.
In such a conventional electric power steering apparatus, the protection against the runaway in the CPU 1 is performed by the drive inhibition circuit 7, while the abnormal condition of the drive inhibition circuit 7 can not be detected. As a result, when the CPU 1 assumes a runaway in an open fault of the transistors 74, 75, it is feared that no protection against the runaway is performed, thereby the system operates in an abnormal condition. Further, when the assistance for the motor is inhibited owing to the runaway in the CPU 1 while the drive inhibition circuit 7 operates in a normal condition, the steering torque increases, but when the CPU 1 returns to the normal condition, the steering torque decreases. As a result, according to the mode of the runaway in the CPU 1, the increase and decrease of the steering torque is repeated, so that the steering feeling deteriorates, and a feeling of uneasiness is produced.
Furthermore, since the detected torque is compensated for phase in the CPU 1, and also is added with the differential value, a situation may arise in which the torque is in the right direction during a detection in a certain moment, while the direction to drive the motor in the rotation is in the left direction. As a result, there is a conflict problem that although the motor would be preferably rotated in the manner described above, the rotation in the left direction or the motor has been inhibited. This situation occurs for the steering in the dead zone and its vicinity, and it is a task to take a countermeasure against the deterioration of the steering feeling.